Computer controlled grinding machine

ABSTRACT

A computer controlled grinding machine programmed so as to control the machine by calculating the wellhead demand positions which takes into account the difference in height between the workpiece axis of rotation and the grinding wheel axis of rotation to produce a height adjusted value for P. The following equation is used to compute the position demand values for a crankpin of a crankshaft, namely: P=(T* cos A)+(R+r) 2 −((T* sin A)+H) 2 ), where P is the eight adjusted) demand position of the grinding wheel at each instant; R is the current radius of the grinding wheel; r is the target radius for the crankpin ( 18 ); T is the throw of the crankpin around the main crankshaft axis ( 16 ); A is the angular position of the crankshaft relative to the start position; and H is the vertical height between the two axes (the height error). The value for P is typically calculated for each of 3600 angular positions during one revolution of the crankshaft.

FIELD OF INVENTION

This invention concerns the grinding of workpieces such as crankpins andthe cam regions of cam shafts, where the grinding wheel performing thegrinding is moved towards and away from the axis about which theworkpiece is rotating so as to maintain engagement with the surfacethereof which is to be ground, as the workpiece rotates around its mainaxis such as in the case of a crankpin which precesses around the maincrankshaft axis, as the latter rotates.

BACKGROUND TO THE INVENTION

The advance and withdrawal of the grinding wheel is normally undercomputer control and with the current development of grinding machines,errors which hitherto were present in ground workpieces have beenlargely eliminated by appropriate programming and secondary errors whichwere previously masked by the larger process errors, have now begun tobe revealed.

Errors such as out of roundness of 1 or 2 microns, can result inunwelcome wear of a final component such as between a crankpin and lowerbig end bearing.

Errors which have already been accommodated, can arise from the varyingheight of the axis of the workpiece region which is being ground (suchas the orbital movement of a crankpin as the crankshaft rotates),relative to the horizontal plane containing the axis about which thegrinding wheel rotates. Typically the throw of a crankshaft is the orderof a few centimeters and there is thus a considerable variation inheight of the axis of the pin relative to the horizontal planecontaining the wheel axis of rotation as the pins are rotated due to therotation of the crankshaft. The grinding wheel is moved towards and awayfrom the crankshaft so as to maintain the grinding contact with thesurface of the pin at all times as the latter is rotated around the maincrankshaft axis, but, assuming that the crankpin axis lies in the samehorizontal plane as the axis of rotation of the grinding wheel, thereare only two points during each rotation of the crankshaft when the pinaxis also occupies that same plane. These are at 3 o'clock and 9 o'clockpositions. At the 12 o'clock and 6 o'clock positions, the pin axis willbe at the maximum displacement above and below the plane and at allintermediate positions, the height of the pin will vary relative to theplane.

The reference to a horizontal plane presupposes that the movement of thegrinding wheel is in a horizontal sense without any divergencetherefrom. This is normally the case but for the avoidance of doubt, itis to be understood that if the locus of the grinding wheel axis as thelatter is moved towards and away from the workpiece, is in a plane whichis not horizontal, the same considerations still apply with regard tothe alignment of the crankshaft axis with the wheel axis, except thatthe “3 o'clock” and “9 o'clock” positions now correspond to when thecrankpin axis lies within the plane containing the path of the movementof the wheel axis.

Computer controlled grinding machines have been programmed to alter thewheelhead demand positions during the crankpin rotation, to compensatefor the errors which can result from the varying height of the crankpinas the crankshaft rotates. Such a machine will be referred to as “of thetype described”.

In the more general case, the main axis of rotation of the crankshaft(or cam shaft as the case may be) will not normally occupy the sameplane as the path of movement of the grinding wheel axis as the latteris moved towards and away from the workpiece, so that there is aconstant height error to be taken into account. Effectively thisintroduces a degree of non-symmetry into the errors arising during therotation of the crankshaft or cam shaft, which would generally besymmetrical if the workpiece axis and grinding wheel axis occupied thesame plane as the path of movement of the grinding wheel axis towardsand away from the workpiece.

It is an object of the present invention to provide a solution to thisproblem.

SUMMARY OF THE INVENTION

According to the present invention in a computer controlled grindingmachine programmed so as to control the machine by calculating thewheelhead demand positions so as to grind the desired workpiece usingappropriate parameters for the workpiece such as roundness, diameter,throw and taper (if required) based on the assumption that the workpieceaxis and grinding wheel axis occupy the same plane as does the path ofmovement of the wheel axis towards and away from the workpiece, whereinthe machine is also programmed to alter the wheelhead demand positionduring workpiece rotation to compensate for errors resulting from thevarying height of the workpiece as the latter rotates, and wherein ademand position value is computed which takes into account thedifference in height between the workpiece axis of rotation and thegrinding wheel axis of rotation for each of a plurality of rotationalpositions of the workpiece around its axis and stored for each position,prior to grinding, and the wheelhead position demand signals employedduring grinding of the workpiece are derived from the stored values.

If the difference in height between a crankshaft workpiece axis and thewheel axis is H, then in accordance with the invention, the demandposition value (P) for each angular position of the workpiece A(measured in the direction of rotation of the workpiece around its mainaxis from a start position) is given by the following equation:

P=(T* cos A)+(R+r)²−((T* sin A)+H)²)  (1)

Where:

R is the current radius of the grinding wheel,

r is the target radius for the crankpin, and

T is the throw of the crankpin around the main crankshaft axis.

Typically the grinding wheel rotates in one sense, e.g. clockwise, andthe crankshaft rotates in the opposite sense, e.g. anti-clockwise, andthe start position is when the grinding wheel is at its furthest (mostrearward) position relative to the crankshaft axis, and the crankpin andcrankshaft axes occupy the same horizontal plane.

Typically the computed value for P is calculated for each of 3600positions during one revolution of the workpiece, ie from A=0 to 2π(which in the case of a rotating crankshaft results in turn in onerevolution of the crankpin about its axis).

Preferably during grinding of the crankpin, the value for P iscalculated at each of a succession of equally spaced apart points intime from the beginning of the grind, by using the appropriate value forP from the stored values of P, or where the angular position of theworkpiece at any instant does not correspond precisely with an angularposition at which a value for P has been stored, a value for P iscomputed by interpolating between the two adjoining stored values for P.

It has been found that a 0.1 millimeter height discrepancy H can resultin a 1 micron roundness error, ie a 1 micron necking of what shouldotherwise be a circular cross-section.

The invention also lies in a computer controlled grinding machine asaforesaid in which the computer is loaded with a program and operated tocalculate and store in a memory the demand position (P) for thewheelhead using and equation for (P) taking account of anynon-circularity or non-concentric rotation of the workpiece, togetherwith any difference in height between the workpiece and wheel axes, foreach of a plurality of positions during one revolution of the workpiece,and the wheelhead feed is subsequently controlled by signals derivedfrom the stored values of (P), during a subsequent grinding of theworkpiece.

The invention also lies in a method of controlling the wheelhead of acomputer controlled grinding machine so as to accommodate errors whichwould arise due to misalignment of the horizontal planes containing thewheel axis and the main axis about which the workpiece is rotated;wherein as a first step, a computer is loaded with a program whichenables the instantaneous demand position for the wheelhead (P) to becalculated for each of N positions of the workpiece for a singlerevolution of the workpiece, and storing the computed value of (P), andas a second step, during grinding of the workpiece, computing the demandposition for the wheelhead at each of a succession of equally spacedapart points in time from the start of grinding, by relating the time tothe angular position of the workpiece and using the N stored values andinterpolating between them where values for P required are intermediatethe values stored for particular angular positions, and as a third stepgenerating a demand position control signal for controlling thewheelhead during grinding using the stored and/or interpolated demandposition values for P.

Preferably in the above method the value of P is recalculated at 1 msintervals during the grinding.

The invention also lies in workpieces when ground using a grindingmachine as aforesaid or a grinding machine operating in accordance withthe above method.

The invention will now be described by way of example with reference tothe accompanying drawing which illustrates in side elevation, a grindingwheel and crankpin workpiece.

In the drawing the grinding wheel 10 rotates about axis 12 and ismounted for fore and aft movement along path 14 to allow the wheel toengage and disengage a workpiece and in the case of an eccentriccomponent such as a crankpin, to allow the wheel to follow the orbitalpath of the pin and maintain grinding engagement between wheel and pin,as the crankshaft containing the pin, itself rotates.

In the drawing, the main axis of the crankshaft is denoted by 16, andthe pin being ground is denoted by 18, with its axis shown at 20.

The pin 16 is situated at the outboard end of a pair of crank-arms oneof which is shown at 22.

The path 14 generally will be horizontal and ideally the crankshaft axisshould lie in the same horizontal plane as the wheel axis 12 and path14.

In the general case, for many different reasons, this will not be thecase, and the perpendicular distance between the plane 24 (containingthe wheel axis 12 and path 14) and the horizontal plane 26 containingthe crankshaft axis 16, is identified by H.

In accordance with the invention, the demand position for the wheel 10at each of a number of rotational positions of the crankshaft iscomputed prior to the commencement of grinding using the formula (2)above. The start position (where A=0) is where the straight line joiningthe crankshaft axis 16 and the pin axis 20 lies in the horizontal plane26, with the pin 18 between the crankshaft axis 16 and the wheel 10.

During grinding, the crankshaft is rotated relatively slowly about itsaxis 16 so that in turn the crankpin is rotated around the crankshaftaxis 16, while the wheel 10 rotates around its axis 12 at a relativelyhigh speed, typically many thousands of revolutions per minute, and isadvanced and retarded relative to the crankshaft so as to remain incontact with the pin in manner known per se.

In a preferred arrangement the demand position P is computed for 3600equally circularly spaced positions of pin 18 around crankshaft axis 16,for a single rotation of the crankshaft between A=0 and A=360° (ie P isrecalculated every {fraction (1/10)}° of a degree of rotation of thecrankshaft) before grinding of the pins commences. During grinding at 1msec intervals from the start of the grind, a value for P is computed byinterpolating between the stored pre-calculated values, dependent in theangle A at each instant. The interpolated values for P are used todetermine the signals required to determine the demand position for thewheelhead, using equation (1) above.

What is claimed is:
 1. A computer controlled grinding machine programmedto grind a workpiece by calculating wheelhead demand positions based onworkpiece parameters obtained by gauging the workpiece and computed onthe assumption that the workpiece axis and grinding wheel axis occupythe same plane as does the path of movement of the wheel axis towards oraway from the workpiece, characterised in that the machine is alsoprogrammed to alter the wheelhead demand positions during workpiecerotation to compensate for errors resulting from the varying height ofthe workpiece as it rotates, such that a demand position value iscomputed which also takes into account the difference in height betweenthe workpiece axis of rotation and the grinding wheel axis of rotationfor each of a plurality of rotational positions of the workpiece aroundits axis, and each demand position value is stored for each of the saidpositions prior to grinding, and the wheelhead position demand signalsemployed during grinding of the workpiece are derived from the storedvalues.
 2. A computer controlled grinding machine as claimed in claim 1adapted to grind a crankpin of a crankshaft, wherein the demand positionvalue (P) for each angular position of the crankshaft A (measured in thedirection of its rotation around its main axis from a start position) iscomputed using the following equation: P=(T* cos A)+((R+r)²−((T* sinA)+H)²) where: R is the current radius of the grinding wheel, r is thetarget radius for the crankpin, T is the throw of the crankpin aroundthe main crankshaft axis, and H is the vertical height between the twoaxes (the height error).
 3. A computer controlled grinding machine asclaimed in claim 2 adapted to grind a crankpin of a crankshaft, whereinthe grinding wheel rotates in one sense and the crankshaft rotates inthe opposite sense and the start position for the grind is when thegrinding wheel is at its furthest (most rearward) position relative tothe crankshaft axis whilst still in contact with the pin, and thecrankpin and crankshaft axes occupy the same horizontal plane.
 4. Acomputer controlled grinding machine as claimed in claim 2 in which thecomputed value for P is calculated for each of 3600 positions during onerevolution of the crankshaft, i.e. from A=0 to 2π.
 5. A computercontrolled grinding machine as claimed in claim 2 wherein duringgrinding of the crankpin, the value for P is calculated at each of asuccession of equally spaced apart points in time from the start of thegrind, by using the appropriate value for P from the stored values of P,or where the angular position of the workpiece at any instant does notcorrespond precisely with an angular position at which a value of P hasbeen stored, a value for P is computed by interpolating between the twoadjacent stored values for P, and the computer is programmedaccordingly.
 6. A computer controlled grinding machine as claimed inclaim 1 adapted to grind a crankpin of a crankshaft, wherein thegrinding wheel rotates in one sense and the crankshaft rotates in theopposite sense and the start position for the grind is when the grindingwheel is at its furthest (most rearward) position relative to thecrankshaft axis whilst still in contact with the pin, and the crankpinand crankshaft axes occupy the same horizontal plane.
 7. A computercontrolled grinding machine as claimed in claim 6 wherein duringgrinding of the crankpin, the value for P is calculated at each of asuccession of equally spaced apart points in time from the start of thegrind, by using the appropriate value for P from the stored values of P,or where the angular position of the workpiece at any instant does notcorrespond precisely with an angular position at which a value of P hasbeen stored, a value for P is computed by interpolating between the twoadjacent stored values for P, and the computer is programmedaccordingly.
 8. A computer controlled grinding machine as claimed in 6in which the computed value for P is calculated for each of 3600positions during one revolution of the crankshaft, i.e. from A=0 to 2π.9. A computer controlled grinding machine as claimed in claim 8 whereinduring grinding of the crankpin, the value for P is calculated at eachof a succession of equally spaced apart points in time from the start ofthe grind, by using the appropriate value for P from the stored valuesof P, or where the angular position of the workpiece at any instant doesnot correspond precisely with an angular position at which a value of Phas been stored, a value for P is computed by interpolating between thetwo adjacent stored values for P, and the computer is programmedaccordingly.
 10. A method of controlling a computer controlled grindingmachine characterised in that prior to the commencement of grinding (1)the computer is loaded with a program to calculate and store in a memorythe demand position P for the wheelhead, using an equation for computingP taking into account any non-circularity or non-concentric rotation ofthe workpiece, together with any difference in height between theworkpiece and wheel axes for each of a plurality of positions during onerevolution of the workpiece, and (2) the wheelhead feed is subsequentlycontrolled by signals derived from the stored values of P duringgrinding of the workpiece.
 11. A method of controlling the wheel head ofa computer controlled grinding machine so as to accommodate errors whichwould arise due to misalignment of the horizontal planes containing theaxis about which the grinding wheel is rotated and the axis about whichthe workpiece is rotated, comprising the steps of (1) loading thecomputer with a program which enables the instantaneous demand positionfor the wheelhead P to be calculated for each of N positions of theworkpiece for a single revolution of the workpiece, (2) storing the Ncomputed values of P, (3) engaging the workpiece with the wheel andduring the grinding of the workpiece computing the demand position forthe wheelhead at each of a succession of equally spaced apart points intime from the start of grinding, (4) relating the time to the angularposition of the workpiece and using the N stored values andinterpolating between them where values for P are required which areintermediate the values stored for particular angular positions, and (5)generating a demand position control signal for controlling thewheelhead during the grinding using the stored and/or interpolateddemand position values for P.
 12. A method a claimed in claim 11 whereinthe value of P is calculated at 1 ms intervals during the grinding.